The present invention relates to the field of electro-acoustical devices, and, more particularly, to mapping vibrational modes thereof.
It is often desirable to measure the oscillatory surface motion of electro-acoustic devices, such as surface acoustic wave (SAW) devices and thin film resonators (TFR), for example. Such measurements may be useful for evaluating different types of materials, new applications, etc. for these devices. One approach for taking such measurements involves the use of an atomic force microscope (AFM), as described in Safar, H. et al., xe2x80x9cImaging of Acoustic Fields in Bulk Acoustic-Wave Thin-Film Resonators,xe2x80x9d Appl. Phys. Lett. 77, 136-138 (2000), for example.
An AFM includes a scanning tip that is placed adjacent a surface of the sample to measure the magnitude of the surface vibrations. While the AFM provides good lateral and vertical resolution, it is limited by a low frequency response (e.g., less than about 20 kHz). That is, at higher frequencies the scanning tip follows the time-averaged height, which varies depending on whether an RF excitation voltage is turned on or off. Thus, with low-frequency amplitude modulation of the RF drive, the signal at the modulation frequency provides high sensitivity to the magnitude of the motion, but no information regarding its phase.
Various forms of optical interferometry have also been used to observe surface motion of electroacoustic devices. For example, interferometric probing of surface acoustic devices has been reported in articles such as Wagner, J. W., in Physical Acoustics, Vol. XIX, Ed. Thurston, R. N. and Pierce, A. D. (Academic, Boston, 1990); Knuuttila, J. V., Tikka, P. T., and Salomaa, M. M., xe2x80x9cScanning Michelson Interferometer for Imaging Surface Acoustic Wave Fields,xe2x80x9d Optics Letters 25, 613-615 (2000); and Drake, A. D. and Leiner, D. C., xe2x80x9cFiber-optic Interferometer for Remote Subangstrom Vibration Measurement,xe2x80x9d Rev. Sci. Instrum. 55, 162-165 (1984). However, such prior art interferometric techniques generally lack high frequency response and high spatial resolution.
In view of the foregoing background, it is therefore an object of the invention to provide a method and apparatus for studying vibrational modes of an electro-acoustic device providing good high frequency response and high spatial resolution to thereby provide three-dimensional imaging of high frequency surface vibrations in electro-acoustical devices.
This and other objects, features, and advantages in accordance with the present invention are provided by a method for studying vibrational modes of an electro-acoustic device including driving the electro-acoustic device to produce at least one vibrational mode therein, collecting phase and amplitude data from the electro-acoustic device using optical interferometry, and mapping the at least one vibrational mode based upon the collected phase and amplitude data. The mapping may include plotting the phase and amplitude data to provide an instantaneous three-dimensional view of the at least one vibrational mode. Moreover, a sequence of the instantaneous three-dimensional views may be constructed to form a motion picture of the at least one vibrational mode. Thus, slow-motion, high-resolution motion pictures representing actual vibrational modes of electro-acoustic devices may be provided according to the present invention.
The electro-acoustic device may be driven with a signal near a resonance frequency of the device. The collecting may include collecting the phase and amplitude data using a Michelson interferometer with an optical detector, for example. An output signal from the optical detector may be homodyned, and the homodyned output signal may be phase-detected to determine the phase and amplitude data. Furthermore, the homodyned output signal may be phase-shifted prior to the phase-detecting to increase signal amplitude.
The collecting may further include directing a sample beam at the electro-acoustic device and comparing a reflected beam with a reference beam. A sensitivity value relating to an alignment error between the reference beam and the reflected beam may be determined, and the amplitude data may be normalized using the sensitivity value. A raster scanner may also be used to step along the electro-acoustic device to provide phase and amplitude data across a surface of the electro-acoustic device.
An apparatus for mapping vibrational modes of an electro-acoustic device according to the invention is also provided. The apparatus may include a signal source for driving the electro-acoustic device to produce at least one vibrational mode therein, an optical interferometer for collecting phase and amplitude data from the electro-acoustic device, and a controller coupled to the signal source and the optical interferometer for controlling the signal source and mapping the at least one vibrational mode based upon the collected phase and amplitude data.